The catabolic pathways of microorganisms are essential for completion of cycles of matter in the environment and the best hope for reclamation of polluted areas. The processes that underlie catabolic evolution are poorly understood. Presumably central events are (i) creation of copies of DNA sequences encoding highly evolved protein structures; (ii) selection of protein functions associated with a novel metabolic process; (iii) modification of DNA sequences so that the protein functions achieve enhanced efficiency. The most obvious protein functions are ligand binding and the catalytic activity of enzymes, but equally important are amino acid sequences associated with protomer-protomer association in oligomeric enzymes and, possibly, the association of different proteins in aggregates of enzymes that catalyze consecutive metabolic reactions. Little is known about the amino acid sequences associated with functions of catabolic enzymes, and it is this gap in knowledge that the research program is designed to fill. The subject of the research is the Beta-ketoadipate pathway, a widely distributed mechanism for utilization of aromatic and hydroaromatic growth substrates. Metabolic reactions included in the study are quite different: cycloisomerization, intralactonic migration of a double bond, lactone hydrolysis, acyl-CoA transfer, and thiolytic cleavage. The available evidence suggests that the enzymes catalyzing respective steps in the pathways are members of enzyme families, groups of divergent enzymes that subject different substrates to similar catalytic reactions. This proposition will be explored by using chemical and genetic techniques to identify amino acid sequences associated with catalytic activity and substrate binding; the results of this study should establish a basis for establishing relationships among enzymes that catalyze similar reactions. By understanding what was selected in metabolic evolution, the amino acid sequences associated with specific functions, we may hope to design rational procedure for directing and accelerating the process.